This invention relates to a method of measuring the mechanical properties of films, and more particularly, to the measurement of thin films under tensile stress.
Numerous methods have been developed to determine the mechanical properties of thin films. The importance of these methods has become well known with the development of integrated circuit fabrication, packaging techniques and the recent growth in solid state sensor applications. Determining these mechanical properties is necessary in assessing the large stresses in thin films as insulating layers, which can cause cracking and adhesive or cohesive failure, leading to component failure. Stress relaxation due to creep can also alter device performance in time.
The ability to fabricate many micromechanical structures depends greatly on the mechanical characteristics of the material. Structures made of materials with tensile stress demonstrate significant performance deviation from the expected performance for the material with zero stress. For example, thin silicon diaphragm pressure sensors, in which the diaphragm is under tension, may exhibit such performance deviation.
With the increasing application of polymer films to microelectronics, it is necessary to study the mechanical properties of these films. To fully characterize the mechanical properties of a thin polymeric film material, the stress, Young's Modulus and Poisson's ratio of the film, as well as ultimate strength of the film should be determined.
Traditionally, the basic technique for measuring the stress in thin films has been to deposit the film of interest on a substrate and measure the stress-induced curvature of the substrate. See R. W. Hoffman, Physics of Nonmetal Thin Films, ed. Dupey and Cachard, Nato Advanced Study Institutes Service B, Vol. 14 Plenum Press, New York, 1976. The in-situ measurement of stress in polyimide films by this "wafer bending" technique was reported in P. Gelderman, C. Goldsmith, and F. Bedetti, "Measurement of Stresses Created during curing and Cured Polyimide Films," In K. L. Mittal, Ed., Polyimides, Plenum Press, New York, Vol. 2, 1984. The deflection or curvature of a beam supported at both ends, a cantilever beam, or a circular plate, can be measured optically by either a laser beam deflection system or by interferometric methods. The deflection can also be measured by capacitance changes or by mechanically probing the surface. See D. S. Campbell, "Mechanical properties of thin films," ed. L. I. Maissel and R. Gland, Handbook of Thin Film Technology, McGraw-Hill, New York, 1970.
J. W. Beams has developed a method whereby a film is deposited on a substrate and a hole is drilled in the substrate without disturbing the film. If the stress in the film is compressive, the film will bow without further pressure. The deflection of the film due to this latent compressive stress can be measured optically. To measure tensile stress using the J. W. Beams approach requires the application of pressure to the film from an external source. See J. W. Beams, eds. C. A. Neugebauer, J. B. Newkirk, and D. A. Vermilyea, Structure and Properties of Thin Films, John Wiley & Sons, Inc., New York, 1954.
These methods require the use of elaborate experimental instrumentation for curvature measurements, or they sacrifice accuracy and completeness to permit the use of simplified measurement methods. Reported in-situ measurement methods may rely on the buckling of microfabricated structures, which are most readily applicable to the measurement of the compressive stresses. See R. T. Howe and R. S. Muller, "Stress in Polycrystalline and amorphous silicon thin films," J. Appl. Phys. 54,4674(1983); H. Guckel, T. Randazzo, and D. W. Burns, "A single technique for the determination of mechanical strain in thin films with applications to polysilicon," J. Appl. Phys. 57,1671(1985).